Lightwave technology is evolving through development stages that in many ways mirror those of semiconductor electronics. The discrete device stage is largely over for lightwave technology, and it is proceeding rapidly to sophisticated levels of integration in what might be regarded as the lightwave phase of hybrid integration. Optoelectronic integrated circuits have the true characteristics of hybrid integrated circuits, and with the development of photonic integrated circuits (PICs) the elements of monolithic semiconductor ICs are emerging. PICs are single substrate ("chip") devices with integrated optical waveguides interconnecting active devices. Prominent among the active devices are lasers, which are the basic elements of most PIC devices.
Problems encountered in designing and fabricating PICs can be divided into two groups: optical engineering problems and optoelectronic-electronic engineering problems. An example of the former is the fabrication of low-loss optical waveguides with the associated constraints on doping types and levels. Another important optical problem is the design and fabrication of low-loss longitudinal coupling between active and passive portions of various guided-wave devices. Another optical problem is the requirement of strictly single-mode guides in the passive sections of the device. Lasers, being non-linear thresholding devices, are often fabricated without strictly fundamental-mode guides. In addition to the dimensional constraints introduced in making buried heterostructure guides strictly fundamental mode, the waveguides typical to most lasers are unsuitable for passive couplers or active waveguide switches due to their sensitive phase characteristics. This can result in large fluctuations in device properties due to the unavoidable fabrication tolerances encountered using presently available technology. For this reason, buried rib guides are preferred in non-thresholding devices such as couplers, switches, and filters. One must then devise a means of coupling between different guide types, i.e., buried heterostructures for lasers and buried rib waveguides for passive waveguides, in a self-aligned and low-loss fashion. It is also required, for low-loss coupling, that the waveguides being coupled are essentially coplanar.
A problem in the aforementioned optoelectronic-electronic category is the requirement for current blocking in lasers. As a consequence of this requirement, the typical laser device in PIC technology has a mesa structure. An example of this type of laser structure is a semi-insulating planar buried heterostructure (SIPBH) type laser as described by B. I. Miller, U. Koren, and R. J. Capik, Electron. Lett. 22,947 (1986). The use of mesa structures recalls a parallel stage of development in semiconductor technology that was, for a time, incompatible with the goal of surface planarity. Not surprisingly, there is now a similar compatibility issue in PIC technology between the planarity requirement of the waveguides, the mesa configuration of the laser, and goal of surface planarity.
The original approach in the art toward these goals was to form the active and passive regions separately and butt couple the two sections. Achieving effective coupling was difficult, and later attempts were made to integrate both sections on a common substrate. One approach along this line was to form the active layer stack over the whole circuit area, then remove the active stack in the passive regions, and regrow the passive layers adjacent the active layers. However, again, acceptable coupling of the waveguides between the two sections was difficult to obtain, and manufacturing yields were low.
In summary, the two planarity objectives described above can be relatively easily realized separately, but to date achieving both in a reliable, and cost effective process, and one that yields a truly integrated planar structure, has been elusive. Since the requirement for a planar optical interface cannot be compromised, the current processing sequence used to form the mesa (laser) structure and the buried rib waveguide of current PIC devices results in a severely stepped surface. A PIC device design, and a processing sequence, that produce a planar device surface, without compromising the device requirements outlined above, would be a major advance in PIC technology.